@menu
* Managing Overlays:: Creating and moving overlays.
* Overlay Properties:: How to read and set properties.
- What properties do to the screen display.
+ What properties do to the screen display.
* Finding Overlays:: Searching for overlays.
@end menu
There are several ways to specify the line spacing for different
parts of Emacs text.
-@vindex default-line-spacing
On graphical terminals, you can specify the line spacing for all
lines in a frame, using the @code{line-spacing} frame parameter
(@pxref{Layout Parameters}). However, if the default value of
@defvar fontification-functions
This variable holds a list of functions that are called by Emacs
-redisplay as needed to assign faces automatically to text in the buffer.
+redisplay as needed, just before doing redisplay. They are called even
+when Font Lock Mode isn't enabled. When Font Lock Mode is enabled, this
+variable usually holds just one function, @code{jit-lock-function}.
The functions are called in the order listed, with one argument, a
-buffer position @var{pos}. Each function should attempt to assign faces
-to the text in the current buffer starting at @var{pos}.
+buffer position @var{pos}. Collectively they should attempt to assign
+faces to the text in the current buffer starting at @var{pos}.
-Each function should record the faces they assign by setting the
-@code{face} property. It should also add a non-@code{nil}
-@code{fontified} property for all the text it has assigned faces to.
+The functions should record the faces they assign by setting the
+@code{face} property. They should also add a non-@code{nil}
+@code{fontified} property to all the text they have assigned faces to.
That property tells redisplay that faces have been assigned to that text
already.
-It is probably a good idea for each function to do nothing if the
+It is probably a good idea for the functions to do nothing if the
character after @var{pos} already has a non-@code{nil} @code{fontified}
property, but this is not required. If one function overrides the
-assignments made by a previous one, the properties as they are
-after the last function finishes are the ones that really matter.
+assignments made by a previous one, the properties after the last
+function finishes are the ones that really matter.
For efficiency, we recommend writing these functions so that they
usually assign faces to around 400 to 600 characters at each call.
@item :script
The script that the font must support (a symbol).
+
+@item :otf
+The font must be an OpenType font that supports these OpenType
+features, provided Emacs is compiled with support for @samp{libotf} (a
+library for performing complex text layout in certain scripts). The
+value must be a list of the form
+
+@smallexample
+@code{(@var{script-tag} @var{langsys-tag} @var{gsub} @var{gpos})}
+@end smallexample
+
+where @var{script-tag} is the OpenType script tag symbol;
+@var{langsys-tag} is the OpenType language system tag symbol, or
+@code{nil} to use the default language system; @code{gsub} is a list
+of OpenType GSUB feature tag symbols, or @code{nil} if none is
+required; and @code{gpos} is a list of OpenType GPOS feature tag
+symbols, or @code{nil} if none is required. If @code{gsub} or
+@code{gpos} is a list, a @code{nil} element in that list means that
+the font must not match any of the remaining tag symbols. The
+@code{gpos} element may be omitted.
@end table
@end defun
efficiently. When Emacs displays an image, it searches the image
cache for an existing image specification @code{equal} to the desired
specification. If a match is found, the image is displayed from the
-cache; otherwise, Emacs loads the image normally.
-
- Occasionally, you may need to tell Emacs to refresh the images
-associated with a given image specification. For example, suppose you
-display an image using a specification that contains a @code{:file}
-property. The image is automatically cached, and subsequent displays
-of that image, with the same image specification, will use the image
-cache. If the image file changes in the meantime, Emacs would be
-displaying the old version of the image. In such a situation, you can
-``refresh'' the image by calling @code{image-refresh}.
-
- In Emacs' current implementation, each graphical terminal possesses
-an image cache, which is shared by all the frames on that terminal
+cache. Otherwise, Emacs loads the image normally.
+
+@defun image-flush spec &optional frame
+This function removes the image with specification @var{spec} from the
+image cache of frame @var{frame}. Image specifications are compared
+using @code{equal}. If @var{frame} is @code{nil}, it defaults to the
+selected frame. If @var{frame} is @code{t}, the image is flushed on
+all existing frames.
+
+In Emacs' current implementation, each graphical terminal possesses an
+image cache, which is shared by all the frames on that terminal
(@pxref{Multiple Terminals}). Thus, refreshing an image in one frame
also refreshes it in all other frames on the same terminal.
-
-@defun image-refresh spec &optional frame
-This function refreshes any images with image specifications
-@code{equal} to @var{spec} on frame @var{frame}. If @var{frame} is
-@code{nil}, it defaults to the selected frame. If @var{frame} is
-@code{t}, the refresh is applied to all existing frames.
@end defun
+ One use for @code{image-flush} is to tell Emacs about a change in an
+image file. If an image specification contains a @code{:file}
+property, the image is cached based on the file's contents when the
+image is first displayed. Even if the file subsequently changes,
+Emacs continues displaying the old version of the image. Calling
+@code{image-flush} flushes the image from the cache, forcing Emacs to
+re-read the file the next time it needs to display that image.
+
+ Another use for @code{image-flush} is for memory conservation. If
+your Lisp program creates a large number of temporary images over a
+period much shorter than @code{image-cache-eviction-delay} (see
+below), you can opt to flush unused images yourself, instead of
+waiting for Emacs to do it automatically.
+
@defun clear-image-cache &optional filter
This function clears an image cache, removing all the images stored in
it. If @var{filter} is omitted or @code{nil}, it clears the cache for
associated memory.
@defvar image-cache-eviction-delay
-This variable specifies the number of seconds an image can remain in the
-cache without being displayed. When an image is not displayed for this
-length of time, Emacs removes it from the image cache.
+This variable specifies the number of seconds an image can remain in
+the cache without being displayed. When an image is not displayed for
+this length of time, Emacs removes it from the image cache.
+
+Under some circumstances, if the number of images in the cache grows
+too large, the actual eviction delay may be shorter than this.
If the value is @code{nil}, Emacs does not remove images from the cache
except when you explicitly clear it. This mode can be useful for